The systems and methods described herein relate to materials and processes for manufacturing membrane assemblies employed to catalyze reactions, and, more particularly, to materials and processes for forming a membrane-electrode assembly of a fuel cell.
Fuel cell technology is set to play a major role in the fuel and power industries in the next few years. In fact, by the year 2010, about 130 gigawatts of fuel cell based generating capacity will be installed in the US and nearly 550 gigawatts worldwide. Platinum-based Fuel Cells Find Commercial Use, Metals Week, Feb. 19, 1996. Most of these installations will be power-plant facilities that act as central power-plants, industrial generators, and commercial/residential generators. These power-plants will employ Phosphoric Acid Fuel Cells (PAFCs), which are the most commercially developed fuel cells, typically use 90 ozs. of platinum in a 500 kw unit, with 80-85% of the metal being recoverable by recycling. With the development of PAFCs, the power industry is poised to provide a source of fuel that is clean, efficient, noiseless and abundant.
Although PAFC technology is well suited for use in power plant fuel cell facilities, their high weight-to-power ratio makes PAFC technology a poor fit for use in vehicles, such as zero-emission vehicles (ZEVs), presently needed to reduce pollution in densely populated areas, such as California, New York, and Italy. For these applications, other types of fuel cells, such as Proton Exchange Membrane Fuel Cells (PEMFCs), pose a better solution. PEMFCs offer a technology that has an acceptable power to weight ratio, and which is also clean, efficient and noiseless. Today, Ballard Power Systems of Vancouver has already installed experimental PEMFCs several on cars and buses in the US.
These experimental systems show encouraging results, and it is now widely recognized that PEMFC technology holds tremendous promise to replace internal combustion engines for vehicular applications. PEMFCs offer superior fuel economy and almost zero emission of air pollutants, and can attain the performance goals of the DOE/industry Partnership for a New Generation Vehicle (PNGV).
However, to deploy PEMFCs into vehicles on a cost-effective basis requires PEFMCs that are low-cost, and reliable. This is an issue for all fuel cells, including PEMFCs, which employ costly metal catalysts, such as platinum and ruthenium, to convert fuel into electrical power, and therefore can have a high material cost. PEMFCs employ a catalyst layer that promotes the reaction of the fuel materials and facilitates the generation of power. Specifically, electrochemical fuel cells use layers of catalyst material to convert fuel and oxidant to electricity and reaction product. For example, fluid reactants can be supplied to a pair of electrodes which are in contact with and separated by an electrolyte. The electrolyte may be a solid or a liquid (supported liquid matrix). PEMFCs generally employ a solid membrane electrode assembly comprising a solid ionomer or ion-exchange membrane disposed between two planar electrodes.
The electrodes typically comprise an electrode substrate and an electro-catalyst layer disposed upon one major surface of the electrode substrate. The electrode substrate typically comprises a sheet of porous, electrically conductive material, such as carbon fiber paper or carbon cloth. The layer of electro-catalyst is typically in the form of finely comminuted metal, typically platinum, and is disposed on the surface of the electrode substrate at the interface with the membrane electrolyte to induce the desired electrochemical reaction.
At the anode, the fuel is oxidized at the anode electro-catalyst layer. At the cathode, the oxidant moves through the porous cathode substrate and is reduced at the cathode electro-catalyst layer. A selective, insulating ion-exchange membrane between the cathode and anode facilitates the migration of protons from the anode to the cathode.
The electro-catalyst is typically provided as a thin layer adjacent to the ion-exchange membrane (see U.S. Pat. Nos. 5,132,193 and 5,409,785). The electro-catalyst layer is typically applied as a coating to one major surface of a sheet of porous, electrically conductive sheet material or to one surface of the ion-exchange membrane.
These electro-catalyst layers compromising platinum and platinum-group elements, both for anode and cathode, are presently a high-cost component of PEMFCs. Studies have shown that the catalyst accounts for $2-3 of the total cost of $15-21/kilowatt. Most of the fuel cell cost is related to the membrane area via current collectors, seals, etc. Accordingly, there is a desire to achieve cost reduction through higher catalyst efficiency by increasing the power per unit area.
Existing techniques for applying catalytic material to the proton exchange membrane produce inefficient loading of catalytic material. For example, as described in the above-identified US Patents, the electro-catalyst layers are commonly formed through liquid slurry infiltration processes that impregnate the platinum into the porous graphite membrane electrode assemblies (MEA) and/or the polymer membrane surface. However, catalyst material is most effective when located proximal the membrane surface and the graphite MEA. The slurry infiltration technique often wastes catalyst because it deposits the catalysts in large chunks deposited too deeply into the electrode material to contribute to electro-catalysis.
Using moderate to low precious metal loading while enhancing catalyst activity and cell performance is the research goal for the PEMFC development community. Substantial progress has been made, but further cost reduction must be achieved in order to enable practical vehicular applications.
Additionally, the efficiency of fuel cells turns in part on the quality of the electrical circuit formed within the cell. However, existing techniques for forming the electro-catalyst layer produce MEAs that have poor electrical connection between the membrane electrolyte and the catalytic material. This poor connection leads to high internal resistance, reducing the power that can readily be produced by these cells. Thus, there is a need for MEAs with lower internal resistance and better electrical connectivity between the membrane and catalytic layer.
In one embodiment, the invention comprises an admixture of an ionomer material, an electrically conductive material and a catalyst. Examples of such materials can include a proton conducting polymeric material, such as Nafion, a conducting material such as graphite and a catalyst such as platinum. The admixture can be employed as an electro-catalyst layer for a PEMFC. To this end, the catalyst can be co-deposited with a spray of Nafion solution onto a surface of a substrate of solid Nafion material. The codeposited Nafion and catalyst can impregnate the near-surface region of the solid Nafion substrate, and build onto the impregnated surface a film of the codeposited Nafion and catalyst. Accordingly, in one embodiment, the codeposited electro-catalyst layer comprises a region at and near the surface of the Nafion substrate which comprises intimately mixed Nafion and catalyst nano-crystallites and which can be between 0.1 and 20 microns thick.
The structure of this electro-catalyst layer avoids the problems with conventional electro-catalyst layers, wherein catalyst particles are deeply embedded within the membrane where they cannot contribute to cell electrochemistry.
In a further embodiment, the catalyst particles can also be formed near the surface and in the pores of the graphite fibers of an electrode assembly. The structure of this electro-catalyst layer similarly avoids the problems with conventional electro-catalyst layers, wherein catalyst particles are deeply embedded within a porous carbon electrode where they cannot contribute to cell electrochemistry.
Optionally, micron-scale conductive spires or granules of a variety of electrically conductive materials can be embedded to enhance gas permeation: hydrogen at the anode; oxygen and water vapor at the cathode.
In a further embodiment, a conducting material, such as graphite, can be deposited as fibers to provide a porous substrate and the nano-particles of catalyst and solution of Nafion can be codeposited with the graphite fibers to provide a porous substrate. The porous substrate can comprise graphite fibers having a conformal layer of catalyst and Nafion.
Other aspects and embodiments of the invention will be apparent from the following description of certain illustrative embodiments.